In main transmission systems and also primary distribution of transmission networks, it is common to provide a variety of switches or cut outs that are operated by a lineman. Commonly, these include a vertical cut out, which is mounted on a pole supporting the transmission line, and an in-line switch which is located horizontally in the transmission line itself. The in-line switch is intended to interrupt the flow of current in the transmission line, whilst the vertical cut out can be used to interrupt a connection to the transmission line, and may be connected to a transformer or another portion of the distribution network.
Both types of switches or cut outs include a combined switch and fuse element, which is pivotally mounted at one end in a bracket. At its other end, it has a rounded cap that can be engaged in a recess of a contact plate, which is under spring pressure. For the sake of uniformity and ease of replacement, electrical utility companies usually require that any switch or cut out design be configured to accept a standard fuse element, which also serves a part of a switch. Then, if a fuse element needs to be replaced, a fuse element from any source, made in accordance with the standard specifications, may be inserted, irrespective of the original manufacturer of the switch or cut out.
Usually, in-line switches and vertical cut outs are treated as distinct, separate components for a variety of reasons, even though they may be required to accept a common fuse element.
Firstly, a vertical cut out has a requirement that it must be mechanically mounted to a vertical pole or support. To this end, it has an insulator that is usually formed from porcelain and provided with a bracket glued centrally to the porcelain body. It is then mounted by this bracket to the pole, etc.
As the cut out is supported by the pole, its weight is not too critical. For this reason, porcelain is usually used since it is cheaper, although it is relatively heavy.
At either end of the porcelain body, connection bars are secured by clamps, to provide the electrical input and output connections.
Brackets are also secured to the connection bars. A first bracket is provided with a pair of side members bearing bracket elements which define U-shaped slots. A fuse element then has pivot lugs that engage these slots for pivotal movement. A second bracket has a spring-loaded connection plate which is urged against the end cap of the fuse.
As detailed below, if the fuse blows, the end cap is released from the connection plate, and the fuse can then freely pivot about its pivot lugs in the bracket elements of the first bracket.
For a cut out, to ensure that the fuse element does not become detached, the bracket elements have the U-shaped slots directed upwardly, and slightly outwardly from the cut out. This ensures that the fuse element is retained if it should blow or fail, while at the same time, enabling a lineman to readily replace the fuse element with a new one.
Now, the requirements for an in-line switch are somewhat different. Here, the switch mechanically is part of the transmission line, strung between supporting towers. Accordingly, its weight is important, and it should be kept as light as possible. It must also be capable of withstanding the tension in the transmission line. For this reason, in-line switches are now often made from EPDM or a silicone polymer insulator. These usually have a fibreglass core to which the mechanical connection bars are secured by clamping at either end. This provides for the necessary mechanical strength, capable of withstanding the tension in the line. Further, unlike the insulator for a cut out, there is no necessity to provide a bracket in the middle, and hence, the insulator for an in-line switch is usually formed as a series of uniformly spaced disc elements, to provide the necessary insulating characteristics.
As an in-line switch is mounted generally horizontally, the bracket elements for holding the pivot lugs of the fuse should be oriented facing upwardly and towards the insulator itself. Then, when the fuse blows or fails, it will swing down and remain pivoted in those bracket elements. Again, it can then be readily exchanged by a lineman for a new fuse element.
In conventional cut outs and in-line switches, the bracket elements are integral with the respective brackets. In view of the different angular orientations of the bracket elements required for the two types of switches, as detailed above, it is then required to form, usually by casting, different brackets for each type of switch or cut out.
Accordingly, for all the foregoing reasons, at the present time it is common to manufacture in-line switches and cut outs as entirely separate components. Bearing in mind that such components can come in a variety of sizes, this places a burden on electrical utility companies to maintain substantial stocks of two different types of devices.